MICROEMULSIONS
Due to their excellent dissolution properties microemulsions have been increasingly used in various technological fields. They are used, for instance as detergents, in oil recovery and as the reaction environment of enzyme catalysts.
Microemulsions are also used as dosage forms of drugs. The British patent publications GB 2222770, 2228198 and 2257395 describe microemulsion concentrates containing cyclosporin which form o/w type microemulsions after addition of water.
Microemulsion denotes a thermodynamically stable and optically isotropic solution which consists of water (or more generally hydrophilic component), oil (or more generally lipophilic component) and surfactant which denotes a surface active substance with an amphilic character. Microemulsions are macroscopically, e.g. when observed visually, homogeneous one-phase solutions. However, if observed on microscopic level, it is noted that they are most heterogeneous. Microemulsions are composed of microscopic continuous domains of water or oil which are separated from one another by a monomolecular layer of the surfactant. The role of surfactant is stabilization of the microemulsion, for instance through decreasing interfacial tension. The microemulsion is of an oil-in-water type (o/w, "oil in water"); a water-in-oil type (w/o, "water in oil") or has a bicontinuous structure (will be explained in more detail later).
The monomeric solubility of the surfactant both in water (hydrophilic component) and in oil (lipophilic component) must be low so that the surfactant could form a microemulsion with the highest possible stability.
The surfactant may be ionic or non-ionic. If the surfactant is ionic, it must have two hydrocarbon chains to form a microemulsion. If the ionic surfactant does not have two hydrocarbon chains, a neutral inorganic salt and a co-surfactant must be added. A short-chain alcohol is often used as the co-surfactant. When a two-chained ionic or a non-ionic surfactant is used, the microemulsion may be formed without any additives. In case the surfactant is too lipophilic to form spontaneously microemulsions in water, a component like ethanol may be added which reduces hydro-philicity of water. Due to its high solubility in water, ethanol is not considered to take part in the formation of a stabilizing interface but to remain in water. Hence it is often called as co-solvent.
FIG. 1 shows the phase diagram of an idealized oil-water-surfactant model system (Kemia-Kemi Vol. 20 (1993) 197). The phase diagram exhibits, in addition to two-phase (W I and W II) and three-phase (W-III) regions, a wide one-phase region (W IV), in which the microemulsions appear. On examining the phase diagram one finds from a region near the water corner an o/w type microemulsion in which there are oil droplets surrounded by the surfactant in water. In a corresponding way, in a region near the oil corner, a w/o type microemulsion is formed. Between these two regions is a region in which the structure of the microemulsion is bicontinuous. It is possible to solubilize water soluble or liposoluble materials, especially macromolecules or parts thereof which have amphilic properties, into water droplets or oil droplets or into bicontinuous structures.